1. Field of the Invention
The present invention relates to an etchant used for fabricating a liquid crystal display device and fabricating method for a liquid crystal display device using the etchant, and more particularly, to an etchant used for forming a gate line and a method for forming a gate line by using the etchant.
2. Discussion of the Related Art
A liquid crystal display panel is one of the most widely used image display devices these days. In particular, a thin film transistor (TFT) liquid crystal display device (TFT LCD), which uses a TFT as a switching device for driving unit pixels, is widely used.
The TFT LCD includes a TFT array substrate on which TFTs as switching devices are arranged in a matrix form, and a color filter substrate having a color filter formed corresponding to the TFT array substrate. Liquid crystal is filled between the TFT array substrate and the color filter substrate.
The TFT array substrate of the liquid crystal display device is where unit pixels are driven by the TFTs, so a process of forming the TFT array substrate is a critical part among processes for forming the TFT liquid crystal display device.
In general, the process of forming the TFT array substrate includes forming a gate electrode; forming a gate insulation layer on the gate electrode; forming a semiconductor layer on the gate insulation layer; forming a source/drain electrode and a data line on the semiconductor layer; forming a passivation layer on the data line; and forming a pixel electrode on the passivation layer.
In particular, the process for forming the gate line and the gate electrode includes depositing a gate metal on a transparent substrate and forming a gate line and a gate electrode through photolithography.
Formation of the gate line will be described in detail with reference to FIGS. 1A through 1D as follows.
First, as illustrated in FIG. 1A, a gate metal 2 such as a copper alloy or aluminum alloy is disposed on a substrate 1 by a sputtering method.
The sputtering method is to deposit metal particles sputtering by a force generated according to collision between a target material and an inactive gas. A metallic thin film is typically deposited through the sputtering method.
As the gate metal, a copper alloy or an aluminum alloy is typically used, and especially, a dual layer of an aluminum alloy and molybdenum is commonly used. The aluminum alloy has excellent electric conductivity and the molybdenum has ohmic contact characteristics with a pad part supplying a gate signal.
After the gate metal layer is formed on the substrate, it is patterned by photolithography to form gate lines and gate electrodes.
That is, as illustrated in FIG. 1B, a photoresist layer 3 is coated at the entire surface of the substrate with the gate metal 2 deposited thereon by a spin coating method, and then, exposed with applying a gate line pattern-formed mask 4.
The photoresist film is a polymer whose bonding structure is changed when exposed to light such as ultraviolet light, and the pattern is formed on the gate metal layer by using such characteristics that the exposed portion is removed or maintained in a developing process.
As illustrated in FIG. 1C, after the photoresist layer 3 is exposed and then developed, a photoresist pattern 5 having the gate line pattern remains on the substrate, and the gate metal layer 2 is etched by applying the photoresist pattern 5 as a mask.
As illustrated in FIG. 1D, the gate metal layer 2 is etched and a gate line 6 is formed through the etching process.
Methods for etching the gate metal 2 includes a wet etching and a dry etching. The wet etching oxidizes the gate metal in a chemical solution to remove it, and the dry etching irradiates ions in a plasma state onto the gate metal to remove the gate metal.
The wet etching has isotropic characteristics that an etching rate is uniform according to an etching direction and the dry etching has anisotropic characteristics that an etching rate is different according to an etching direction.
Many thin layers are formed on the gate line and the gate line needs to be formed in a tapered shape in order to prevent cutting of the thin layer. Thus, to make the gate line have the tapered shape, the wet etching exhibiting the isotropic etching characteristics is used to etch the gate line.
In the related art in which the dual layer of the aluminum alloy and molybdenum is used as the gate metal and a mixed solution of H3PO4, HNO3 and CH3COOH is used as an etchant, each etching rates of the aluminum alloy layer and of the molybdenum layer are different in the etchant, so the tapered shape is deformed.
FIGS. 2A and 2B illustrate a method in which a dual layer of aluminum alloy layer and a molybdenum layer is used as the gate metal layer, and the gate line is formed using a mixed solution of phosphoric acid (H3PO4), nitric acid (HNO3) and acetic acid (CH3COOH) as an etchant.
FIG. 2A illustrates the aluminum alloy layer 21 and the molybdenum layer 22 having different etching rates. In this case, the aluminum alloy layer 21 is etched by phosphoric acid of the etchant and the molybdenum layer 22 is etched by nitric acid of the etchant. Because the reactivity between the aluminum alloy layer 21 and phosphoric acid is greater than that between the molybdenum layer 22 and nitric acid, the molybdenum layer 22 at the upper side of the aluminum alloy layer 21 is larger than the etched aluminum alloy layer 21 after etching process, as illustrated in FIG. 2A.
Thus, in order to make a perfectly tapered form, the wet etched-molybdenum layer 22 needs to be etched one more time by dry etching. Then, the molybdenum layer 22 has such a tapered form as the aluminum alloy layer 21.
FIG. 2B illustrates the stacked molybdenum layer 22 and aluminum alloy layer 21 tapered by the dry etching.
After the etching process is finished, the photoresist remaining on the substrate is removed and washed to form gate lines.
To sum up, the gate line forming process may include a step of depositing the gate metal on the substrate; a step of forming the photoresist layer pattern on the gate metal; a step of performing a wet-etching by applying the photoresist layer pattern as a mask; a step of additionally dry-etching the wet etched-gate metal; and a step of removing the photoresist layer and performing a washing.
In the process of fabricating the TFT array using the dual layer of aluminum alloy layer and the molybdenum layer, if the related art etchant is used, the wet etching is performed and then the dry etching is to be performed additionally, causing a process delay. In addition, since an equipment for the dry etching is required, an expense is increased.
Moreover, as for the pattern of the gate line by using the conventional etchant, since the side tilt angle of the tapered form is so large that cutting is caused in forming a thin layer on the gate line.
FIG. 3 is a photograph taken by electron microscope showing a gate line etched by using the related art etchant. As shown, a profile of the gate line has a sharp tilt.
The profile of the gate line is sensitive to the cutting of various thin films formed on the gate line, so it is critical for the gate line to have a gentle, rather than a sharp, profile in order to prevent cutting.